Looped, phased array laser oscillator

ABSTRACT

A phase-locked fibre bundle laser oscillator made up of one or more active fibre loops as illustrated. The single mode fibre has a doped core surrounded by cladding and has its ends positioned in a common plane forming a two core aperture which is optically polished to a twentieth wavelength. Fibre end faces are coupled by index matching material to a single, partially transmitting, dielectric mirror and the oscillator is side pumped via the output of an array of laser diodes.

FIELD OF THE INVENTION

This invention relates to a laser oscillator system whose basic buildingblock is a side excited looped, lasing ion doped fiber laser oscillatorwith a single mirror, index matched to the output ends of said fiberloop. The invention is scaleable to any practical power level simply byadding together the said looped fiber laser oscillators to form aphased-locked bundle of said oscillators all index matched to a singleoutput mirror. The invention has applications in the medical, industrialand defence fields where a scaleable source of diffraction limited,laser beam energy is required, in particular where its high structuralflexibility is an advantage.

SUMMARY OF PRIOR ART

Prior art fiber bundle laser oscillators possessed two mirrored endswhich could move independently of each other and said ends had to beoptically treated separately. Prior art fiber bundle laser oscillatorswere excited either via one of the said two ends of via laser mediainserted into said fibers in the core where undoped fibers were used.The optical excitation of prior art fiber bundle laser oscillators wasvery restricted because the diode pumps had to be matched to the verysmall fiber cores at the ends of said bundle. In the case of lasersources being inserted into a bundle of undoped fiber, a complexassembly procedure had to be followed which significantly increased thecost of said laser oscillator systems.

The present invention overcomes the complex and expensive assembly andoperational costs of prior art fiber bundle based laser oscillatorsystems by providing simple side excitation of a large number of laserion doped, looped fibers simultaneously. In applications where only anunswitched laser beam is required, that is, in applications demandingonly a continuous or pulsed laser output beam with no selectiveswitching of the individual fibers, the present invention provides amajor advance on prior art fiber bundle laser systems providing a costeffective means of generating a laser beam which is scaleable in powersimply by adding more fiber loops to the bundle and providing additionaldiode excitation sources for the side excitation of said bundle via asimple, direct, side illumination of said fiber bundle.

The present invention is relatively simple to manufacture in comparisonwith prior art laser oscillator systems. A single mode, optical fiberdoped with the appropriate lasing ion, is simply wound onto a rotatingdrum and automatically moved by the stacking action which provides acoherently packed reel of optical fibers. Once the thickness of the saidoptical fiber layers have built up on the drum, which has to be of adiameter which is far greater than the thickness of the fiber layers,the said layers of coherently packed, and the doped optical fibers arecut along the axis of rotation of said drum so that the required loopedfiber laser bundle is automatically produced as the fiber layers aremoved from said drum. By bringing the two free ends of said fiber bundletogether into a common aperture and optically polishing said apertureand index matching it to a single laser output mirror, the invention canbe completed in a relatively short time as far as the manufacture of oneof its major components is concerned. The invention is scaleable to anypractical power level because its size simply depends on the size of thedrum used in the manufacture and the length of fiber wound onto saiddrum.

BACKGROUND OF THE INVENTION

These are two approaches that can be taken to the generation of a laserbeam within laser oscillators either the laser oscillator's activemedium is excited as a whole or the active medium can be split into alarge number of sections, the laser beam output of each of the saidsections being then phased-locked together to produce a single beamequivalent to that emitted by the single section medium laseroscillator.

Two development avenues have resulted in the techniques necessary togenerate a single laser beam by phase-locking the output beam of a largenumber of smaller laser beam emitting apertures, namely, fiber bundlelaser arrays and arrays of semiconductor lasers. The fundamentaldifference between these two development avenues is the fact that thefiber laser aperture array is a cold array, involving only thetransmission of the laser light through the said aperture whilst thesemiconductor array aperture is a hot aperture because up to 75% of theelectrical energy into said aperture is deposited as heat energy with inthe said aperture, only about 25% of said electrical energy beingconverted to laser light within the diode array. Although phase-lockingof semiconductor arrays is now well established, no reports are to handthat suggest such diode arrays have coherently phased-locked on a largescale, that is large diode arrays phased-locked in pockets across theaperture, a process that leads to a severe degrading of the structure ofthe emitted laser beam.

The inventor has pioneered key aspects of fiber bundle basedphased-array lasers since 1963 when a team set up by the BritishGovernment consisting of microwave radar pioneers and laser physicistswas stationed at The Royal Radar Establishment, Malvern, UK, todetermine the avenues along which conventional radar techniques could beused to develop laser radar. One of the avenues studied was that for thetransfer of microwave, phased-array radar techniques into the opticalregion and optical fiber bundles were assessed experimentally for thistask by the inventor at the Royal Radar Establishment as early as 1963.However, these early experimental tests revealed that a helium-neonlaser beam was converted into "non-laser light" as soon as it enteredthe fiber bundles available in those days and the development of fiberbundle based phased-array laser radars was held up until single modeoptical fibers became available some seventeen years later during thelate 1970's. A key process in phased-array laser radar utilizing bundlesof single mode optical fibers was published in 1979 (Hughes and Ghatak,applied Optics, U.S.A., 1979).

Early phased-array laser radar patents by the inventor were classifiedby the US patent Office in June 1983 and remain classified. However, acommercially orientated phased-array, fiber bundle laser oscillatorconsisting of undoped optical fibers was patented in the United Statesin 1987 (U.S. Pat. No. 4,682,335 Hughes, July 1987). However, the priorart, fiber bundle based, phased-array lasers were difficult to assemblecompared to the relative simplicity of the present invention which lendsitself to simple, but highly effective mass production techniques.

The first of our looped, neodymlun doped fiber lasers was constructedand operated under contract from the assignee by YORK TECHNOLOGY Ltd ofSouthampton, UK in 1988. However, the individual looped fiber lasers inthe 20 bundle system manufactured by YORK under contract to the assigneewere side excited with a 830nm laser diode output coupled into the coreof each of the looped laser oscillators in the bundle of saidoscillators via a commercially available optical coupler manufactured byYORK TECHNOLOGY Ltd for the optical communications market.Unfortunately, such couplers are expensive and are not appropriate inthe low cost, unswitched, diode excited looped, fiber laser bundlerbased laser oscillator of the invention. For example, when one packs thefiber bundle so that the fibers are in contact with each other, theyrepresent a solid block of glass in most respects, in particular fromthe viewpoint of direct optical excitation. The fact that the fibersused to date have a 5 micron diameter doped core and an 80 microndiameter cladding does not affect the optical pumping because the volumeof the fiber cores being excited is the beam as if the excitation lightwas coupled into each individual fiber with an array of very expensiveoptical couplers.

To achieve coherent phase-locking of the present invention is a muchsimpler process than is generally thought. For example, the length ofthe fiber loops in the individual fiber laser oscillators need not bethe same, the critical length is that corresponding to a 360 degreephase change, that is the one corresponding to a complete laserwavelength within the fiber core. If the compacted end face of theinvention is optically polished to say a twentieth wavelength and indexmatched to the output mirror surface also polished to a twentiethwavelength, then the effective optical path of all the loopsirrespective of their individual physical lengths, will be equal to atwentieth of a wave and well suited for coherent phase-locking of thefiber end array. It should also be noted that the supermode of operationresulting in the coherent phase-locking of the array, is paralleled overthe number of loops and is not seriesed over the total length of theloops. In other words, the mean length of the loops represents the fiberlength over which the supermode has to be maintained. Furthermore, thebroad gain curve of the doped glass fibers used means that thesupermodes in different looped fibers can differ in wavelength. In otherwords, supermode pulling effects in individual fibers can alsocontribute to coherent phase-locking across the aperture of theinvention.

To minimize the cost of manufacturing the invention, it is an advantageto be able to use the optical fiber manufactured worldwide for opticalcommunications needs. These fibers generally have a core diameter ofabout 5 microns with a cladding diameter of about 125 microns. Suchthick cladding is necessary to minimize the optical signal loss from thesignal transmitting core of the fiber. Such thick cladding also protectsthe said fiber core from mechanical damage, a base 5 micron diametercore being extremely fragile. By design there is not transverse opticalcoupling between such fiber cores in an array of such fibers so that thephase-locking process has to be achieved either by reflective orrefractive coupling, a combination of both and some transverse opticalcoupling via the index matching material be it liquid or solid. Theinvention can be Q-switched using techniques known in the art. Inparticular a thin film of slid dye switch placed within the indexmatched material can accomplish such switching of the invention.Experiments using the invention have also shown that its output laserbeam can be modulated by modulating the excitation light.

OBJECTS OF THE INVENTION

It is an object of the invention to produce a phased-array laseroscillator by stacking together in a bundle, a number of optically sideexcited looped fiber laser oscillators whose ends faces forming theoutput aperture array are all indexed matched to a single, opticallypolished output mirror.

Another object of the invention is to spread out the fiber laser bundleso as to minimize heating effects due to heat generated within saidbundle during the lasing process.

A further object of the invention is to arrange the bundle of loopedfiber lasers such that they represent a long solid slab of fibersallowing for their efficient, direct optical excitation.

Yet a further object of the invention is to fully utilize the thicklyclad optical fiber manufacture for world wide optical communicationapplications, with appropriate laser ion doping of its core.

It is an object of the invention to provide a flexible body for theinvention which can be attached to such items as robotic arms withoutthe use of prior art articulated arms.

SUMMARY OF THE INVENTION

A better understanding of the invention may be obtained from thefollowing considerations taken in conjunction with the drawings whichare not meant to limit the scope of the invention in any way.

FIG. 1 shows the fundamental building block of the invention, namely,the looped fiber laser oscillator with its phased-locked output beam andits side excitation.

FIG. 2 shows the configuration of the invention after a large number ofthe looped fiber lasers have been bundled together and the side opticalexcitation is provided by two arrays of laser diodes and an interveningmirror which reflects any incident pump light back into the fiberbundles.

FIG. 3 shows a preferred embodiment of the invention with the sideexcitation taking place near one end leaving the other, output beam end,highly flexible and capable of being hand held or machine mounted.

FIG. 4 shows the manner in which portions of the looped fibers of theinvention can be polished to produce a rectangular cross-sectioncladding in which the circular core of the mass produced optical fiberis embedded, being adequately protected in the process. Thisconfiguration of the optical fibers allows for the close packing of saidfibers in a manner that allows efficient side coupling of the excitationlight directly into cores of said fibers.

FIG. 5 shows the manner in which excitation can be coupled into saidfiber core of a given looped fiber via one or more optical fibersconnected to laser diode arrays emitting the pump light.

FIG. 6 shows reflecting coupling means used to phase-lock the fiber endtransmitters forming the inventions output aperture.

FIG. 7 shows he refractive coupling that can be used to phase-lock thefiber end transmitters from the output aperture of the invention.

FIG. 8 shows the multicored fibers that can be used to increase thenumber of individual laser fiber core transmitters in the outputaperture.

FIG. 9 shows the drum used to coherently stack the doped optical fiberas it emerges from the fiber puller.

FIG. 10 shows the way in which layers of the coherently stacked dopedoptical fiber is cut to form the fiber bundle of the invention.

FIG. 11 shows the manner in which the looped fiber bundle comes off thedrum in the configuration required for mass producing the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, numeral 1 indicates a single looped fiber laser oscillator,the building block of the present invention. Numeral 2 indicates thecore of the single mode optical fiber, which, for example in the case ofneodymlun doping is 500 parts per million, in the fiber core, which fora single mode fiber is about 5 microns in diameter. The absorption pathof the 830nm excitation light along the core of such fibers is over twometers. Numeral 3 indicates the cladding surrounding the single modefiber core. In the case of mass produced single mode optical fiber foroptical communications, the diameter of the cladding is normally 125microns or 25 times core diameter. The thicker the fiber cladding thefewer fiber ends that can be packed into the output aperture of theinvention. It is a relatively simple process o mass produce single modedoped fiber with a cladding diameter of 80 microns but an extremelydifficult task to mass produce such optical fiber with a claddingdiameter of much less than 80 microns, particularly if the cladding hasto be etched.

in FIG. 1, numeral 4 indicates the two core aperture of the singlelooped fiber laser oscillator which is optically polished to a twentiethwavelength. Numeral 5 indicates the index matching material to couplethe fiber end faces 4 to an optically polished laser output mirrorindicated by numeral 6. Numeral 7 indicates the optically polished andpartially mirrored surface of substrate etalon 6. The outer surface ofthe etalon, indicated by numeral 8 can either be antireflection coatedor coated to form a Fabry-Perot resonator. Numeral 9 indicates thephased-locked output of the single looped fiber laser oscillator of theinvention. Numeral 10 indicates the optical radiation used to sideexcite the looped fiber 3.

In FIG. 2, numeral 11 indicates the looped fiber laser bundle of theinvention. Numeral 12 indicates the laser diode arrays used to generatethe excitation light for the side pumping of the bundle 11. Numeral 13indicates a mirror reflecting the excitation light that may be incidenton it back into bundle 11 to increase the excitation efficiency of theinvention. Numeral 14 indicates the power supply for the laser diodearray 12 whilst numeral 15 indicates the input for power supply 14.

In FIG. 3, numeral 16 indicates the extended portion of fiber laserbundle 11 which allows fore greater flexibility of the hand held portionof the invention which is located in the casing indicated by numeral 17.Numeral 18 indicates the focused output beam of the invention necessaryfor applications in the medical and industrial fields where high beamintensities are required for cutting, for example.

In FIG. 4, numeral 19 indicates the fiber cladding polished into arectangular configuration. Numeral 20 indicates the doped fiber core ofcircular cross-section embedded in the polished cladding 19. The cores20 can be stacked together in a close packed array which can be veryeffectively optically excited via the side excitation indicated bynumeral 21.

In FIG. 5, numeral 22 indicates the core of an optical fiber which cancouple excitation light from a remotely sited diode stack into the dopedfiber core.

In FIG. 6, numeral 23 indicates the laser beams emerging from the fibercore ends indicated by numeral 24 and reflected from core to core viareflection off mirror 7.

In FIG. 7, numeral 25 indicates the outputs of fiber core ends 24 beingrefractively coupled from core to core via the Farbry-Perot etalonindicated by numeral 26.

In FIG. 8, numeral 27 indicates one of the fiber cores contained withincladding 3 of fiber 1 in its multicored configuration.

In FIG. 9, numeral 28 indicates the single mode optical fiber being fedinto its pulling station so as to be coherently wound on the rotatingdrum indicated by numeral 29 to build up fiber layers indicated bynumeral 30.

In FIG. 10, numeral 31 indicates the coherently packed fiber layers ondrum 29 being cut along the axis of rotation of drum 29.

In FIG. 11, the mass produced bundle of looped fiber lasers after beingtaken off drum 29.

The invention has wide application in the medical, industrial anddefence fields where a laserbeam of easily scaleable power output isrequired from a flexible body. By selecting the appropriate diode pumpand fiber core doping it is possible to operate the laser over a widerange of output wavelengths form the visible to the infa-red regions ofthe electromagnetic spectrum. By frequency doubling, tripling andquadrupling the fundamental output frequency, and by controlling thetemperature of the excitation diode lasers it is possible to tune thefrequency shifted outputs of the invention further.

The power output of a single looped fiber laser can be as high as 10milliwatts and 10,000, 80 micron thick clad fibers can be packed into anaperture of a square centimeter. This implies that a continuous laserbeam power of 100 watts per cm² of the aperture can be emitted by thelaser using mass produced optical communications fiber.

Under pulsed operation, peak powers of our fiber cores per squarecentimeter can be extracted out of a single core using short durationpulses so that their is ample scope for high peak power outputs form theinvention as a whole provided it is Q-switched or modelocked usingtechniques which are well known in the art.

Modification may be made to the above teachings by those skilled in theart without departing from the scope and spirit of the invention.

I claim:
 1. A scaleable phased locked single aperture, single mirrorlooped fiber laser bundle oscillator with the optically polished ends ofthe said fiber laser oscillator loops positioned to form the said singleoutput aperture and optically matched to the said single, opticallypolished mirror which partially transmits at the laser wavelength, saidfiber laser loop oscillators being optically excited using arrays ofsemiconductor light sources.
 2. A laser oscillator system as claimed inclaim 1 where the optical excitation is restricted to the region halfway along the said loop of said fiber laser bundle oscillator allowingthe said flexible single output aperture end of said oscillator to befree for ease of handling and mounting onto industrial work stations. 3.A laser fiber laser as claimed in claim 1 where two sides of the fibercladding has been polished to form a cladding of rectangularcross-section whose width matches the diameter of the fiber core ofcircular cross-section embedded in said polished cladding.
 4. A seriesof closely packed optical fibers as claimed in claim 3 where the coresare in close contact with each other in a given plane allowing foreffective coupling of the excitation light output from the laser diodeexciters into a closed packed array of fiber cores.
 5. A bundle ofpolished optical fibers as claimed in claim 3 wherein the excitationlight propogating in one of the said fiber cores is coupled into thelaser fiber core which is positioned as near as possible to the saidfiber core in which the excitation light propagates.
 6. A fiber laseroscillator as claimed in claim 1 where the fiber cladding has severallaser fiber cores embedded in it to inclose the excitation efficiency anto increase the density of the output core transmitter in the outputaperture for a given number of clad optical fibers.